Plant roots are organs adapted to accumulate water and nutrients from the soil and to provide these necessary ingredients for optimal growth and development of the entire plant. Plant roots also carry out specialized functions that contribute to overall plant yield and in case of root or tuber crops, constitute the essential plant yield. Root growth and development have been reviewed (See: Aeschbacher, R. A, Schiefelbein, J. W. and Benfey, P. N. The Genetic and Molecular Basis of Root Development. Annu. Rev. Plant Physiol. Plant Mol. Biol., 1994, 45, 25-45; Schiefelbein, J. W., Masucci, J. D. and Wang, H. Building a Root: The Control of Patterning and Morphogenesis During Root Development. Plant Cell 9, 1997, 1089-1098). While meristem maintenance and proliferative growth of roots is determined by cell cycle regulation and cyclin expression or plant hormones such as ethylene and auxin can enhance root growth (See: Boerjan, W., Cervera, M. T., Delarue, M., Beeckman, T., Dewitte, Wl, Bellini, C., Caboche, M., Van Onckelen, H., Van Montagu, M. and Inze, D. Superroot, A Recessive Mutation in Arabidopsis, Confers Auxin Overproduction. Plant Cell 7, 1995, 1405-1419) additional regulatory factors appear also to be necessary for new root growth.
Root encounters with soil environmental conditions determine plant productivity and a well developed root system functions in nutrient and water uptake and determines to a significant extent plant yield. The function of the roots is profoundly influenced by soil nutrient composition and any toxins as well as abiotic and biotic environmental stress. Thus, inhibition of shoot growth with continued root growth has been considered as a morphological adaptation to water stress or salt stress (See: Creelman, R. A., Mason, H. S., Bensen, R. J., Boyer, J. S. and Mullet, J. E. Water deficit and abscisic acid cause differential inhibition of shoot versus root growth in soybean seedlings. Plant Physiol., 1990; 92, 205-214). Increased root mass may also play an important defensive role in metal toxicity, since reduced shoot expansion and yield are considered to be secondary from inhibition of root growth and nutrient accumulation (See: Larsen, P. B., Kochian, L. V. and Howell, S. H. Al Inhibits both shoot development and root growth in als3, an Al-sensitive Arabidopsis mutant. Plant Physiol., 1997, 114, 1207-1214). Improved root growth and development thus can enhance overall plant productivity and appears to be a desirable trait for manipulation in plants.
The present work is an outgrowth of early efforts to develop crop plants with improved salt tolerance that included the regeneration of plants after selection of salt-tolerant cells in culture (See: Winicov, I. Characterization of salt tolerant alfalfa (Medicago sativa L plants regenerated from salt tolerant cell lines. Plant Cell Reports, 1991; 10, 561-564; Winicov, I. Characterization of rice (Oryxa sativa L) plants regenerated from salt-tolerant cell lines. Plant Sci., 1996; 113, 105-111) coupled with identification of genes differentially regulated in the salt tolerant cells and plants (See: Winicov, I. and Bastola, D. R. Salt tolerance in crop plants: New approaches through tissue culture and gene regulation. Acta Physiol. Plant., 1997; 19, 435-449). Transgenic plants have been constructed in a number of other laboratories to over-express single genes, known to be up-regulated by salt/drought stress in prokaryotes or plants (See: Holmberg, N. and Bulow, L. Improving stress tolerance in plants by gene transfer. Trends in Plant Sci., 1998; 3, 61-65). However, the molecular mechanisms by which plants can acquire improved long term salt tolerance and maintain their productivity are still not understood and may involve the regulation of many genes (See: Winicov, I. New molecular approaches to improving salt tolerance in crop plants. Annals of Botany 1998; 82, 703-710), since salt tolerance has been considered to be a quantitative trait (See: Foolad M. R., Jones R. A. Mapping salt-tolerance genes in tomato (Lycopersicon esculentum) using trait-based marker analysis. Theor. Appl. Genet., 1993; 87, 184-192). Thus, the identification of regulatory genes that can influence the expression of other genes in a specific manner could be particularly useful in manipulating not only plant growth, but also enhance their tolerance to a variety of biotic and abiotic environmental stress conditions.